Obama and Kerry try to drum up support for Syria plan

The president and secretary of state worked the phones to try and rally support for intervention in Syria. NBC’s Kristen Welker reports.

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>>>tonight is presenting new evidence that
sarin gas
was used in a
chemical attack
in syria, as it tries convince
congress
to authorize a
military strike
there it's unclear if lawmakers will approve the request when they return next week from recess, but already, the decision to ask for support has had ripple effects both here and around the world. we have complete coverage tonight and we begin with nbc's kristen welker at the
white house
. kristen, good evening.

>> reporter: carl, good evening to you. the decision to give
congress
a vote is a political gamble by
president obama
. one
white house
official tells me the strategy moving forward will be to flood the zone and that effort started today with the chief of staff, vice president and president making a flurry of
phone calls
to lawmakers and other top officials holding a rare holiday briefing today on
capitol hill
. with the president's credibility on the line, secretary of state
john kerry
fanned out on the sunday shows, revealing new evidence, further bolstering the case for
military action
.

>>hair samples and blood samples have tested positive for signatures of sarin. so this case is building and this case will build.

>> reporter: administration continued its push with an unusual classified briefing to lawmakers on
capitol hill
today, more than 100 turned out, despite the holiday weekend.

>>i think members of
congress
were divided.

>> reporter: the developments cap a stunning turn of events. the president spent a week indicating a
military strike
was imminent, sending out secretary kerry to make the case and ordering navy warships to be ready. but on saturday, mr. obama hit the pause button.

>>i will seek authorization for the
use of force
from the
american people
's representatives in
congress
.

>> reporter: according to senior administration officials, despite the public ramp-up,
president obama
debated seeking congressional support all week and without a broad international coalition, feeling isolated, aides say it was the no-vote in the
british parliament
that largely swayed him.

>>the nos have it.

>> reporter: the president went for a walk with the chief of staff on the
white house
grounds and decided he wanted
congress
to be on record and share the responsibility. he called a meeting in the
oval office
that lasted two hours with some of his closest advisers expressing shock and opposition, but by saturday, they were on board. today, secretary kerry indicated did he not feel undermined by the president and put on a
united front
.

>>david, let me be pre blunt. i do not believe the
congress of the united states
will turn its back on this moment.

>> reporter: but this morning, lawmakers previewed the contentious battle to come.

>>this is a clear failure of leadership.

>>this is a
national security
issue. this isn't about
barack obama
versus the
congress
.

>>there's weakness here on the part of the president.

>>i think it's a mistake to get involved in the syrian
civil war
.

>>if i had to
walk over
to the capitol right now, i would pull out my voting card and vote no.

>>i would vote yes because the whole world is watching and so is iran.

>>congress
overrules a decision of the
president of the united states
on an issue of
national security
, that could set a catastrophic precedent in the future.

>> reporter: analysts say the president is taking a
political risk
.

>>if he gets fired down, it could be the talk of a lame duck presidency.

>> reporter: senator patrick leahy
told reporters today the president's resolution will be amended because it's too broad. officials here say they aren't surprised. the
white house
outreach continues. the president will hold
conference calls
and meetings with lawmakers this week. carl?